In recent years, silicon nitride, Si.sub.3 N.sub.4, has been investigated extensively as a high strength ceramic material. Hot pressing has allowed Si.sub.3 N.sub.4 to be prepared in forms which are very strong at room temperature (as exhibited by a transverse rupture strength, greater than 100,000 psi) provided suitable additives are incorporated into the material before hot pressing as hot pressing aids. Unfortunately, the hot pressing technique is unsuitable for making items of complex shape. Silicon nitride articles of complex shape may, however, be made by the process of reaction sintering, in which the desired shape is prepared (by cold pressing, slip casting, etc.) from silicon powder and then nitrided and sintered by heating in a nitrogen atmosphere. Articles prepared in this manner are, however, not very strong (10,000 to 40,000 psi in transverse bending).
Silicon nitride is believed to occur in two forms, .alpha. and .beta., both having structures based on the Si.sub.3 N.sub.4 tetrahedra sharing corners. It is not certain, however, that either one is not stabilized by impurities, oxygen in the case of the .alpha. form or cations in the case of the .beta. form. It has now become possible, as known to the art, to react alumina with silicon nitride at high temperature to form a material with the same structure as .beta. - Si.sub.3 N.sub.4, but an apparently somewhat larger unit cell. Ceramic materials prepared from a combination of silicon nitride and alumina are generally referred to by the name SIALON, an acronym for Si, Al, O and N. It is presumed that aluminum atoms replace some of the silicon atoms and oxygen atoms replace some of the nitrogen atoms in .beta. - Si.sub.3 N.sub.4. It is not clear just how much Al.sub.2 O.sub.3 can be accomodated in the .beta. - Si.sub.3 N.sub.4 structure, since at least one additional phase, of uncertain composition and structure, is often formed as a result of the reaction. But it is now known how an intimate mixture of silicon nitride and alumina can be sintered to form a high strength ceramic in the absence of applied pressure, as well as by hot pressing. In most work currently being reported on the SIALON system, high strengths are not often achieved and, when they are, it is only by hot pressing.